Lead metal removal by recycled alum sludge

In this study, alum sludge was recycled using a chemical precipitation process to promote the removal of lead metal in wastewater. To make the process more cost-effective, two different pH ranges were suggested for daily operation, depending on the involvement or otherwise of an aluminum regeneration process at a later stage. If aluminum regeneration was expected, an elevated pH (11.6) was suggested to optimize both the aluminum regeneration and lead removal. Lead hydroxide coprecipitated with alum sludge in alkaline conditions, where the high pH would also facilitate the alkaline regeneration of alum at a later stage. With the use of recycled alum sludge (RAS) in the coagulation process, lead removal rates increased from 79% to 96–98% with 100–180 mg/l of RAS, while the required fresh alum dosage was reduced from 175 mg/l to 50–12.5 mg/l only. If no fresh alum is added to the system, the RAS can still remove up to 94% of lead at low recycling doses (75–100 mg/l), which suggests that the sweep-floc mechanism is crucial to the sludge reuse process. However, if the regeneration of aluminum is not required, a lower initial pH (10.5) should be used to decrease chemical costs.